Class that has responsibility for comparing Nodes and notifying a
DifferenceListener of any differences or dissimilarities that are found.
Knows how to compare namespaces and nested child nodes, but currently
only compares nodes of type ELEMENT_NODE, CDATA_SECTION_NODE,
COMMENT_NODE, DOCUMENT_TYPE_NODE, PROCESSING_INSTRUCTION_NODE and TEXT_NODE.
Nodes of other types (eg ENTITY_NODE) will be skipped.